Many radio receivers provide users with a convenience feature for automatically searching for broadcast frequencies in a band of interest and selecting only those broadcast signals received with sufficient quality to be considered "listenable" for sustained audio output. This feature, known as search tuning in a "seek", or scan (with a truncated period of sustained audio output) operating mode is typically engaged by pressing a button, which then causes the tuner to automatically advance to the next frequency on which a station may broadcast, and evaluate whether a "listenable" broadcast is being received. If so, the tuner remains on that frequency. Otherwise, the tuner advances to the next available frequency.
Evaluating whether a received broadcast is listenable, is a task whose ease of implementation varies with the method used to modulate and demodulate the information contained on the radio frequency carrier. The evaluation of Amplitude Modulated (AM) information tends to be more formidable than most. Typical measures of "listenability" include received signal level and tolerance of the Intermediate Frequency (IF).
Received signal level is typically indicated by a DC voltage derived from the rectification of the IF signal level. The DC voltage amplitude is proportional to the amplitude level of the carrier frequency received to which the radio is tuned. Presumably, the stronger the received signal level, the better its sound quality delivered to the listener, until overload occurs for very strong signals.
Tolerance of the intermediate frequency is typically indicated using a counter that compares a pulse representation of the IF signal derived from the received signal to its theoretical value for a properly tuned station. Presumably, if this frequency is within a close tolerance, for example, within about 1 KHz for a 450 Khz signal, to the theoretical, then the tuned frequency contains a valid station.
A prominent characteristic of AM signals, however, is their poor noise immunity. Radiation in the AM band often occurs from sources other than the transmitter broadcasting the desired signal. These sources often radiate broadband energy, such that many frequencies are influenced. Since the energy typically varies in amplitude, the amplitude modulation detector in the receiver reproduces the interference as audible noise. This noise typically occurs in the same frequency range as the desired audio, and is therefore difficult to distinguish. As a result, the measures of listenability previously described may be influenced by undesired radiation, thereby making it difficult to determine acceptance thresholds with which the received signals is to be compared for selection of a listenable broadcast.
Another characteristic of AM band signals is the effect of ions above the atmosphere. These ions change as the sun goes down in a way that causes AM band signals to reflect off the ionosphere, thus increasing the distance from the transmitter that the signals can be received. The level of these long distance signals often varies slowly with time and can therefore interfere intermittently with closer stations broadcasting at the same frequency, or those broadcasting at harmonics of the distant transmitter's frequency. Therefore, thresholds of listenability used for daytime reception will likely produce stops on unlistenable stations at night. While previously known receivers may include a nighttime switch that changes the threshold level between two values, such a switch is not responsive to other environmental conditions.